The flexible food packaging industry is currently experiencing relatively substantial growth due to the increasing popularity of ready-to-eat and microwaveable foods and drinks which appear to spontaneously multiply on our grocers' shelves. The flexible food packaging utilized in these popular products is typically comprised of a laminate of various polymeric films and/or metal foils. The utilization of a combination of polymeric films and metal foils allows a package to be designed so as to take advantage of the various desirable properties of the films and foils such as permeability, heat resistance and the like. The various films and foils are typically combined into a laminate by utilizing an adhesive to bond the layers of materials together.
Adhesives utilized for the manufacture of these high performance, flexible food packaging laminates have previously been based on conventional two-part urethane adhesive systems. These urethane adhesives are relatively difficult to work with since they require the mixing of two separate parts and since they typically have a limited potlife or storage capability. The urethane adhesives also typically utilize undesirable volatile organic solvents and require an extended cure cycle in order to fully develop the ultimately desired properties.
Although not previously disclosed as being useful for laminating adhesives, various photocopolymerizable compositions containing epoxy compounds have been previously described in the patent literature. For example, U.S. Pat. No. 4,256,828 discloses photocopolymerizable compositions containing epoxides, an organic material with hydroxyl functionality, and a photosensitive aromatic sulfonium or iodonium salt of a halogen-containing complex ion. The epoxide can be a cycloaliphatic epoxide while the hydroxy-functional materials can be alkanols, alkylene glycols, polyoxyalkylene glycols and triols, hydroxyl-terminated vinyl acetate copolymers, hydroxyl-terminated polyvinylacetal resins, hydroxyl-terminated polyesters, hydroxyl-terminated polylactones and hydroxyl-terminated polyalkadienes.
Another example of a photocopolymerizable epoxy-based composition is described in U.S. Pat. No. 4,593,051. This patent describes photocopolymerizable compositions based on epoxide and polymer/hydroxyl-containing organic materials. The epoxide can be a cycloaliphatic epoxide resin while the polymer/hydroxyl-containing organic materials can be polyether polyols or a polymer-polyol dispersion prepared by the free-radical polymerization of acrylonitrile or a mixture of acrylonitrile and styrene in a polyoxyalkylene polyol containing unsaturation.
Various previously described photocopolymerizable compositions such as those disclosed above have been found by the present inventors to be ineffective when utilized as a laminating adhesive for flexible packaging materials. Specifically, the previous photocopolymerizable compositions have been found to not provide a level of bond strength or green strength (the ability to adhere and firmly hold together the materials to be bonded prior to cure) acceptable for an effective utilization in flexible packaging applications.
A need therefore exists for an adhesive composition which can be utilized as a laminating adhesive in a one-part system so as to provide acceptable adhesion and potlife or storage capability. Such an adhesive composition should also exhibit sufficient green strength and should avoid the use of volatile organic solvents.